First , we should make out the root of the tree....
You can use a DFS to make it into son-brother tree(My En

void Zh(int p)
{
  f[p]=1;
  for(int i=0;i<d[p];i++)
    if (!f[next[p][i]])      
      {
      bro[next[p][i]]=son[p];
      son[p]=next[p][i];
      Zh(next[p][i]);                       
      }
}

TreeDP
Make maxt[0][p][m] the max-eat of point p's right brothers and sons below , which uses m steps and should back to p .
Make maxt[1][p][m] the max-eat of point p's right brothers and sons below , which uses m steps and need not back to p

To every p ,first consider without p (Give the right of the steps directly to his right brothers) 
Next we consider the condition that p must be gotten
Condition 1 . Only get p's sons below
Condition 2 . Only get p's right brothers
Condition 3 . get both p's sons and brothers...

Now,what is difficult if maxt[0] and maxt[1]

maxt[0] :must back to p.....Simply make the right of steps -2 when go to the sons below and the right brothers...
maxt[1] :need not back to p.....Simply make the right of steps -2 when go to the right brothers and -1 when go to the sons below...

If you get through these two questions....You will make it easily..

I really don't know whether I can make myself out....
If there're some erros..... Contact me 


Source Code

Problem: 2486  User: forsona 
Memory: 620K  Time: 422MS 
Language: G++  Result: Accepted 

Source Code 
#include"iostream"
#define INF -99999999
using namespace std;
int d[110],next[110][110],son[110],bro[110],c[110],f[110],n,k,maxt[2][110][210],r;
void Zh(int p)
{
  f[p]=1;
  for(int i=0;i<d[p];i++)
    if (!f[next[p][i]])      
      {
      bro[next[p][i]]=son[p];
      son[p]=next[p][i];
      Zh(next[p][i]);                       
      }
  memset(maxt,-1,sizeof(maxt));      
}
void Inp(void)
{
  int i,j,k;
  memset(d,0,sizeof(d));
  for(i=1;i<=n;i++)
    scanf("%d",&c[i]);
  for(i=0;i<n-1;i++)
    {
    scanf("%d%d",&j,&k);
    next[j][d[j]++]=k;
    next[k][d[k]++]=j;
    } 
  memset(bro,0,sizeof(bro));
  memset(son,0,sizeof(son));    
  memset(f,0,sizeof(f));  
  Zh(1);
  
}
int Loop(int b,int p,int m)                          
 //b==0  back to root     b==1  no need to root
{
  int i,mt;
  if (maxt[b][p][m]!=-1)
    return maxt[b][p][m];
  if (!p)
    return maxt[b][p][m]=0;  
  if (m==0)
    return maxt[b][p][m]=max(Loop(b,bro[p],m),c[p]);  
  if (b==0)
    {
    mt=Loop(0,bro[p],m);
    if (m>=2)
      {
      mt=max(mt,c[p]+Loop(0,bro[p],m-2));
      mt=max(mt,c[p]+Loop(0,son[p],m-2));
      }
    for(i=0;i<=m-4;i++)  
      mt=max(mt,Loop(0,bro[p],i)+Loop(0,son[p],m-4-i)+c[p]);
    return maxt[b][p][m]=mt;        
    }
  else
    {
    mt=Loop(1,bro[p],m);       
    mt=max(mt,c[p]+Loop(1,son[p],m-1));
    if (m>=2)
      mt=max(mt,c[p]+Loop(1,bro[p],m-2));
    for(i=0;i<=m-4;i++)
      mt=max(mt,Loop(1,bro[p],i)+Loop(0,son[p],m-4-i)+c[p]); 
    for(i=0;i<=m-3;i++)         
      mt=max(mt,Loop(0,bro[p],i)+Loop(1,son[p],m-3-i)+c[p]);          
    return maxt[b][p][m]=mt;       
    }  
}
int main()
{
  while(scanf("%d%d",&n,&k)!=EOF)
    {
    Inp();
    r=Loop(1,1,k);
    printf("%d\n",r);                             
    }
  return 0;  
}